Annual rhythms: photoperiodism, dormancy and hibernation
Annual (circannual) rhythms
- An annual or circannual rhythm has a period of about one year, matching the seasons.
- It controls events that happen once a year: flowering, seed germination, breeding, moulting, migration, dormancy and hibernation.
- Like other rhythms it is endogenous — animals kept in constant conditions for long periods still show annual cycles of breeding condition and body mass, though the period drifts from exactly 12 months.
Why day length is the cue
-
The zeitgeber for almost all annual rhythms is photoperiod — the length of the day.
-
The reason is reliability, and this is the key idea of the whole page.
-
Compare the possible seasonal cues:
- Temperature varies unpredictably. A warm week in winter or a cold snap in spring is common, so temperature is a noisy signal.
- Rainfall and food availability vary between years, and are consequences of the season rather than predictors of it.
- Day length is set by the Earth's axial tilt and orbit. It is identical every year, cannot be disturbed by weather, and changes fastest around the equinoxes — precisely when seasonal preparation must begin.
-
So day length is the only cue that reliably predicts what is coming rather than reporting what has already arrived. Responding to a predictor allows preparation; responding to a consequence guarantees being late.
-
Photoperiodism is the response of an organism to the length of day or night.
Photoperiodism in plants
-
Plants detect photoperiod using the pigment phytochrome, which exists in two interconvertible forms:
- Pr absorbs red light (660 nm) and is converted to Pfr.
- Pfr absorbs far-red light (730 nm) and is converted back to Pr.
- Pfr is the active form — it is what triggers responses.
-
The mechanism:
- In daylight, red light dominates, so phytochrome is converted to Pfr and accumulates.
- In darkness, Pfr slowly reverts to Pr.
- The longer the night, the more Pfr reverts, so the amount of Pfr at dawn measures how long the night was.
-
Plants are classified by their response:
- Long-day plants flower when days are longer than a critical length — that is, when nights are shorter than a critical length. They flower in spring and early summer.
- Short-day plants flower when days are shorter than a critical length — when nights are longer. They flower in autumn.
- Day-neutral plants flower regardless of photoperiod, usually responding to age or size instead.
-
The crucial refinement: it is the length of uninterrupted darkness that matters, not the length of the day.
- A flash of light in the middle of the night resets phytochrome to Pfr and makes a long night register as two short nights.
- This prevents a short-day plant from flowering and allows a long-day plant to flower — strong evidence that the plant is measuring the night.
Annual responses in plants
- Flowering at the right time so that pollinators are active and seed ripens before winter.
- Dormancy — growth suspended, metabolic rate reduced, tissue protected against frost.
- Shortening days trigger the hormone abscisic acid, which closes stomata, induces bud dormancy and inhibits growth.
- Deciduous species withdraw nutrients from leaves and shed them before winter, avoiding frost damage and water loss when soil water is frozen.
- Vernalisation — a requirement for a period of cold before flowering can occur.
- This prevents an autumn-germinating seedling from flowering immediately in mild autumn weather, ensuring it flowers only after a genuine winter has passed.
- Seed dormancy — seeds shed in autumn do not germinate until spring, so seedlings do not emerge into frost.
Annual responses in animals
- Breeding season timing. Photoperiod triggers hormonal changes that develop the gonads over weeks.
- Long-day breeders (many birds) begin gonadal development as days lengthen in spring, so young hatch when insect food peaks.
- Short-day breeders (sheep, deer) mate in autumn, so that after a long gestation the young are born in spring.
- Migration. Departure is triggered by photoperiod, along with hyperphagia — a period of intense feeding that lays down fat before departure.
- Moulting into denser plumage or coat before winter.
- Hibernation and torpor — a controlled reduction in metabolic rate, body temperature and heart rate.
- Torpor is short-term, often daily; hibernation is prolonged and seasonal.
- Preparation — fat deposition, finding a den — takes weeks, which is why it must be triggered by an anticipatory cue.
- True hibernation is rare in New Zealand's mild climate, but wētā enter cold torpor, and some alpine wētā survive being frozen solid through winter.
Selective advantage
-
Anticipation is the whole point. Every annual response takes weeks of preparation — gonads to develop, fat to deposit, nutrients to withdraw from leaves. An organism that began when conditions changed would be weeks late and would miss the season entirely.
-
Matching offspring to the food peak.
- A bird that starts developing gonads when days lengthen has eggs laid and chicks hatching exactly when insect abundance peaks. Chicks then grow fastest and more fledge.
- A bird that waited for warm weather would produce chicks after the peak had passed, and more would starve.
-
Avoiding lethal conditions.
- Dormancy and leaf fall before the first frost prevent tissue damage. Frost-damaged tissue is a total loss of the resources invested in it.
- Hibernating through winter avoids the period when energy expenditure would exceed the energy obtainable from available food.
-
Synchrony within the population.
- A shared cue means individuals become reproductively ready at the same time, so mates are available. An individual ready a month early would find none.
- Synchronised seed release also swamps seed predators, so a greater proportion of seed survives.
Worked Example
Worked Example
A native shrub is grown under controlled photoperiods. Flowering is recorded.
| Treatment | Light | Dark | Flowered? |
|---|---|---|---|
| A | 14 h | 10 h | No |
| B | 10 h | 14 h | Yes |
| C | 10 h | 14 h, interrupted by a 1-minute flash of red light at the midpoint | No |
| D | 10 h | 14 h, flash of red light immediately followed by far-red light | Yes |
Identify the type of plant, explain how it measures the season, and explain why this response provides a selective advantage.
Answer:
Identifying the plant. The plant flowers in treatment B (10 h light, 14 h dark) but not A (14 h light, 10 h dark), so it flowers when days are short and nights are long. It is a short-day plant, and it flowers in autumn.
What treatment C shows. Treatment C has exactly the same total light and dark as B, yet flowering is prevented. The only difference is that the dark period was interrupted. So the plant is not measuring the total amount of light or dark — it is measuring the length of continuous darkness. A short-day plant is more accurately a long-night plant.
How the mechanism explains this. The plant detects photoperiod using phytochrome:
- In daylight, red light converts phytochrome to Pfr, the active form, which accumulates.
- In darkness, Pfr slowly reverts to Pr. The longer the uninterrupted night, the more reverts, so a low Pfr level signals a long night — and therefore autumn.
- In treatment B the 14-hour night allows Pfr to fall below the threshold, so flowering is triggered.
- In treatment C the flash of red light converts phytochrome straight back to Pfr. The plant now experiences two separate 7-hour nights, neither long enough for Pfr to fall below the threshold, so flowering does not occur.
What treatment D confirms. Following the red flash with far-red light converts Pfr back to Pr, undoing the effect of the flash. Flowering occurs again. This red/far-red reversibility is decisive evidence that phytochrome is the photoreceptor responsible — no other pigment shows this behaviour, and the result could not be explained by the plant simply detecting "light".
Why the response provides a selective advantage.
Flowering must be timed so that the seed produced has the best chance of surviving, and day length is the only cue that is identical every year. Temperature varies unpredictably — a warm spell in early autumn would mislead a plant relying on it — whereas photoperiod is set by the Earth's tilt and orbit and cannot be disturbed by weather.
Timing flowering by photoperiod means:
- Pollinators are present. Flowers open when the insects that pollinate them are still active, so a high proportion of ovules are fertilised. Flowering after insect activity ceased would waste the entire investment in flowers and nectar.
- Seed ripens before winter. Development takes a fixed number of weeks, so starting at a reliable date ensures seed is mature and shed before frost, rather than being killed part-formed.
- Population synchrony. All individuals receive the same photoperiod cue, so they flower together. For a cross-pollinated species this is essential — an individual flowering a month early would have no compatible pollen available and would set no seed at all.
- Preparation time. Flower development takes weeks. Only an anticipatory cue allows the plant to begin early enough to be flowering at the right moment.
Individuals whose flowering is timed accurately therefore produce more viable seed, and their offspring inherit the same critical night length — so the response is maintained and refined by natural selection.